Many systems used in the fabrication of silicon devices are essentially systems for controlling the temperature of a silicon workpiece while some process is carried out. For example, dopants are deposited on or implanted in a masked silicon workpiece at prescribed temperatures and the workpiece is then heated to a prescribed higher temperature to provide a desired degree of diffusion or anneal. Similarly, layers of various additional materials such as oxides, nitrides and metals are deposited at prescribed temperatures and subjected to various heat treatments. Thus stations for carefully controlled heating of silicon workpieces are important parts of silicon device fabrication equipment.
A typical silicon workpiece heating station comprises a workpiece support, apparatus for measuring the temperature of the workpiece, and controllable heating elements, such as infrared bulbs, for heating the workpiece. Electronic circuitry receives the measured temperature and controls the heating elements to achieve the desired temperature as a function of time.
As silicon processing technology advances, the performance of conventional workpiece heating stations becomes less satisfactory. A major problem is presented by ongoing efforts to reduce the workpiece temperatures to below 500.degree. C. in many processing steps.
Conventional heating stations use thermocouples or optical pyrometers to measure the temperature of the workpieces. While thermocouples are effective over a wide temperature range, they must attach to the workpiece for accuracy. Such attachment is costly, time-consuming and may contaminate the workpiece. Optical pyrometers need not be attached, but they have reduced accuracy at lower temperatures and lack the required accuracy at temperatures below 500.degree. C.
Various efforts have been made to develop new temperature measuring devices for such applications. All have problems measuring and controlling workpiece temperature with the desired accuracy over the desired temperature range.
One such effort is to improve pyrometers by use of a ripple technique. This technique takes advantage of the thermal modulation of the AC current which powers the heating lamps. The pyrometer measures the oscillating component of light emitted from the workpiece. With this technique, the temperature of a workpiece can be controlled to an accuracy of 12.degree. C. at temperatures near 1100.degree. C., and pyrometers using the technique are effective to near 600.degree. C. with decreasing accuracy. But lower operating temperatures and higher accuracy are required.
A second class of efforts is directed to measure semiconductor workpiece temperature by measuring variation in the reflectivity of the semiconductor near its optical bandgap energy. This band gap is the energy below which the semiconductor is substantially transparent. In some semiconductors, near the bandgap energy, the amount of reflected light changes rapidly with wavelength, and the energy gap shifts slightly as temperature changes. Thus it has been proposed to measure temperature by shining light of such wavelength on a semiconductor and measuring the reflectivity. This method works satisfactorily with direct bandgap semiconductors such a GaAs, but it does not work with silicon because silicon has an unusually weak texture in its reflectivity near its band gap. This is due to the fact that silicon is an indirect bandgap material.
A third group of efforts is to measure the temperature of a semiconductor by the change in the transmitted light near the optical band energy. While the technique will work for pure silicon, it does not work for silicon workpieces modified by the deposition of films on the workpiece surface because many of the materials normally deposited for integrated circuits are significantly absorbing or opaque. Thus, this approach is undesirably sensitive to deposited materials. Accordingly, there is a need for improved apparatus for measuring and controlling temperature of silicon workpieces during the fabrication of silicon devices.